In the petroleum industry, boreholes are drilled through earth formations by drilling systems so that hydrocarbons can be extracted from subterranean reservoirs. The drilling system for such drilling of operations comprises a drillstring that extends from the surface down to the bottom of the borehole.
FIG. 1 illustrates an example of a drilling system for operation at a well-site to drill a borehole through an earth formation. The well-site can be located onshore or offshore. In this system, a borehole 311 is formed in subsurface formations by rotary drilling in a manner that is well known. Similar drilling systems can be used drilling boreholes in directional drilling, pilot hole drilling, casing drilling and/or the like.
A drillstring 312 formed from a length of drillpipes 360 and a bottomhole assembly (BHA) 300, is suspended within the borehole 311. The BHA includes a drill bit 305 at its lower end. The surface system includes a platform and derrick assembly 310 positioned over the borehole 311, the assembly 310 including a rotary table 316, kelly 317, hook 318 and rotary swivel 319. The drillstring 312 is rotated by the rotary table 316, energized by means not shown, which engages the kelly 317 at the upper end of the drillstring. The drillstring 312 is suspended from a hook 318, attached to a traveling block (also not shown), through the kelly 317 and the rotary swivel 319 which permits rotation of the drillstring relative to the hook. As is well known, a top drive system could alternatively be used to rotate the drillstring 312 in the borehole and, thus rotate the drill bit 305 against a face of the earth formation at the bottom of the borehole.
In the example, the surface system further includes drilling fluid or mud 326 stored in a pit 327 formed at the well site. A pump 329 delivers the drilling fluid 326 to the interior of the drillstring 312 via a port in the swivel 319, causing the drilling fluid to flow downwardly through the drillstring 312 as indicated by the directional arrow 308. The drilling fluid exits the drillstring 312 via ports in the drill bit 305, and then circulates upwardly through the annulus region between the outside of the drillstring and the wall of the borehole, as indicated by the directional arrows 309. In this well-known manner, the drilling fluid lubricates the drill bit 305 and carries formation cuttings up to the surface as it is returned to the pit 327 for recirculation.
The BHA 300 may include a set of collars 363 for attaching to the drillstring 312, a logging-while-drilling (LWD) module 320, a measuring-while-drilling (MWD) module 330, a rotary-steerable system and motor, and drill bit 305.
The LWD module 320 may be housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at 320A. The LWD module may include capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. The LWD module may include a fluid sampling device.
The MWD module 330 may also be housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drillstring and drill bit. The MWD tool may further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. The MWD module may include one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, a rotation speed measuring device, and an inclination measuring device.
Drilling an oil and/or gas well using the drilling system depicted in FIG. 1 may involve drilling a borehole of considerable length; boreholes are often up to several kilometres vertically and/or horizontally in length. As depicted, the drillstring comprises a drill bit at its lower end and lengths of drill pipe that are screwed/coupled together. A drive mechanism at the surface rotates the drill bit against a face of the earth formation to drill the borehole through the earth formation. The drilling mechanism may be a top drive, a rotary table or the like. In some drilling processes, such as directional drilling or the like, a downhole motor that may be powered by the drilling fluid circulating in the borehole or the like, may be used to drive the drill bit.
The drillstring undergoes complicated dynamic behaviour in the borehole during the drilling procedure, which complicated behaviour may include axial, lateral and torsional vibrations as well as frictional and vibrational interactions with the borehole. Simultaneous measurements of drilling rotation at the surface and at the bit have revealed that while the top of the drill string rotates with a constant angular velocity, the drill bit may rotate with varying angular velocities.
Strong rotational and axial resonances localized within the BHA can cause problems while drilling. Non-linear interactions, either at the bit or through wall contact, can lead to complicated high-amplitude vibrations at the resonant frequencies. Because most of the energy of these resonances is localized within the BHA, direct control from the surface can be difficult.